LC tank oscillators have been used for communication systems, particularly for high frequency LO (local oscillator) signal generation for microwave or radio frequency apparatus. An LC tank circuit includes inductors and capacitors arranged to oscillate by exchanging current or voltage between inductors and capacitors with a finite frequency. Since small resistance in the inductors and capacitors tends to dissipate the energy in the oscillator, the LC tank loses energy and eventually stops oscillating. A negative resistance usually generated by active devices included in the oscillator compensates for energy dissipation caused by resistance and sustains the oscillation. However, even though the resistance is compensated in this manner by the active devices, the resistance still degrades the oscillation quality by affecting the oscillation amplitude, phase noise and oscillation purity (i.e. whether the oscillation has a pure sine wave shape). One measure of quality of oscillation is called the “Quality Factor.” The Quality Factor is defined as the ratio of the energy stored in the LC tank to the energy dissipated in the resistor per oscillation cycle.
Thus, it is desirable that inductors used in an LC tank oscillator have minimum resistance. Unfortunately, on chip inductors generally have high resistance. Low quality oscillation generally results due to the substrate resistance and Ohmic resistance of the metal used in on chip inductors. Therefore, the phase noise performance of oscillators using on-chip inductors is poor and generally not suitable for high quality wireless devices such as cellular phones or satellite communication equipment.
What is needed is an oscillator design that will enable on chip inductors to be used for such high quality wireless devices.